Yeah you could add them but you said plaintext so I thought you didn't want to include them. It's also a good idea to limit the length too. So in summary restrict each variable to a whitelist of characters, limit the length and finally escape the output depending on the context.

If you'd like to add plain html to your site I've been working on a method in JavaScript to do that:-
http://code.google.com/p/htmlreg/source/browse/trunk/HTMLReg/samples/test.php